In Dark Frontier, Voyager travelled with transwarp - which, according to Prof. Hansen, is similar to quantum slipstream.

You have it backwards. In "Hope and Fear," Seven of Nine reported in her log that slipstream was similar to Borg transwarp. Her father, Professor Hansen, was assimilated decades earlier and had no knowledge of slipstream's existence. Slipstream is not mentioned in "Dark Frontier" at all; the only episodes that reference it are "Hope and Fear," "Timeless," "Think Tank," and "Inside Man."

To my knowledge, Voyager accessed the alien network only twice before it collapse, to send the Doctor and receive Starfleet's response.

Well, two times for the Doctor's outgoing and return journeys in "Message in a Bottle," and a third time for receiving the Starfleet transmission in "Hunters."

However, it can be hacked: In the STO timeline, Sela used in 2403 to contact Hirogen in the Beta Quadrant to ally with them.

But the network was knocked out in "Hunters." Well, they said it was "disabled," but it was treated as a permanent disabling, considering the show never used it again. Did STO assert that it was repaired at some point?